Supports for and reinforcements of above-grade structures are often referred to as “piles” or “pilings”. Also, such structures are often referred to as “supports”. The terms piles, pilings, and supports will for purposes of this invention be regarded as synonymous. Further with regard to these terms, the term piling in its general sense is a structure that gives a known vertical response to a load exerted directly on top of it. The familiar construction of a beach-side pier relies on this property. The pier structure is simply built so as to be tied to the top of the pilings and is supported at its underside by the pilings.
A support of the type contemplated by this invention is below grade and utilizes as part of its composition the soil whose volume it replaces. In-situ pilings are often spoken of as supports”. These terms are regarded as synonymous and will be used interchangeably. The structure of an in-situ piling contrasts with conventional supports, which are generally lengths of tree trunk grown and prepared, or cast concrete shapes manufactured elsewhere or at this site from foreign material.
Conventional pilings and supports preferably are driven or otherwise placed in direct contact with bedrock or other supporting structures. If the bedrock or other structure is too deep, then reliance for support is placed on the “skin friction” between it and the surrounding soil. The inherent advantage of the continuity of an in-situ piling with its surrounding soil is evident. Its equivalent of skin friction is a merger of the surrounding soil with the enhanced material of the in-situ piling-a very substantial difference which is advantageous whether or not the in-situ piling reaches bedrock or other supporting structures or relies on the engagement (for convenience sometimes referred to as skin friction) of the in-situ piling with its surroundings. In fact, the in-situ piling does not have a skin in the same sense as a conventional piling has.
Conventional pilings, although in use for many decades, are limited in their utility. For one thing they must usually be made elsewhere, carried to the work site, and then somehow inserted into the ground. Familiar examples are percussive pile drivers, vibrational insertion systems, and excavation of shafts into which the piling is inserted. These are all very costly, but if an optimally consistent structure, whose unit properties are known, constant, and maximum literally from inch-to-inch is required, these are often the best available solution, and they are in fact widely used.
The costliness of conventional pilings is an economic limitation on their application, but for installations with critical properties they are in widespread use. However there exist many applications for affordable piling technology which, while the ultimate unit strength of cement or wood pilings is not necessary, still require predictable and suitable structural properties. These would often be used instead of conventional pilings, provided their properties were sufficient, predictable, and affordable. The term “affordable” includes such considerations as the cost of materials, the cost of manufacture of the piling structure, and the cost of transportation and labor and its installation, all related to a sufficient strength.
This situation has not gone unnoticed. A first thought is to manufacture the piling on-site, or near to it. If this means casting a cement piling at a location adjacent to the site, the savings would be obscure at best, because all of the ingredients (usually cement and aggregate) would still have to be brought to the site and a manufacturing process set up.
This invention proposes the use of an in-situ piling (or column) in which only water and binder (often cement or lime or both) would have to be brought to the site. A boring-mixing type tool would be brought to the site, there to dig into the ground, and mix water and binder into the existing earth material. The intended result would be a sub-grade column on which an above-grade structure could reliably rest, existing in the earth without having been manufactured off-site and without having been driven into place, and without the need for an open shaft to be formed.
Several principal arrangements have been proposed for this purpose. Perhaps the oldest is often called the “dry method”, in which a tool bores into the ground and while so doing adds a binder to the existing material which reacts with water already existing in the soil, with it to form a cementitious column. The shortcomings of the dry method are evident. There may not be enough water already there for the purpose. Still, the dry process has been widely used, and still is in use to this day.
Another process, commonly called the “wet method”, has also been widely used. In this method the binder is provided as a slurry of water and binder, which is injected into the ground while the tool bores into it. There are considerable disadvantages in this system including wastage of binder, variability of the properties of the piling from depth to depth, and clean-up costs, which can be very large.
There have been previous efforts to overcome the disadvantages of the wet and dry methods. One is familiarly called the “Modified Dry Method” in which the soil is preconditioned with water as the tool moves down, and cement is added on the way up. This is the subject of applicant's U.S. Pat. No. 5,967,700 issued Oct. 19, 1999, title “LIME/CEMENT COLUMNAR STABILIZATION OF SOILS”.
Another previous effort is European patent No. 0411 560 BI granted May 4, 1994 to Trevi S. P. A. (referred to herein as the “Trevi” patent). This is an effort to produce an in-situ piling with only sufficient water provided for “humidification”. As will become evident, this patent leads away from the concept of this invention.
The above criteria do not attend to the effects on the ultimate product of two substances which are ultimately involved, namely air and water. The only known way to inject the dry binder into the sub-surface region is by entraining it in a pressurized stream of air. Unavoidably this means injecting very substantial volumes of air into the formation. There, unless it can escape, it can form air pockets in the piling which reduce the strength of the column itself.
Even worse, it adds to the volume of the mix, and a large heaving of surrounding soil will be formed. This is a very serious consequence. In railroad construction, for example, it has been found to shift the track bed sideways by as much as 10 feet. To counter this effect, a large weight is often placed on top of the heaved adjacent areas. For example a thick layer of rock which compresses the intubated soil and surroundings holds the ground down and allows the air to escape. After this has occurred, the rock is removed. This is a very costly procedure. It would be far better not to have the heave in the first place.
This invention provides a means to dispose of the air on an on-going basis. It is an object of this invention to provide an in-situ piling which causes at most a minimal amount of expansion while being mixed.
The way to do this is to provide an environment in which the air will readily percolate out. In this invention this means sufficient fluidly (or fluidization) of the mixture. In turn, in this invention this involves the use of excess water for fluidization.
In processes that involve manufacture of cementitious solids by reaction with water, it is general knowledge that there is an amount of water necessary for complete hydration of the binder (the stoichiometric amount). It is also known that water in excess of this amount may and usually does result in a cured solid with less unit strength than if only the stoichiometric amount had been used. Finally it is possible to “kill” the cement with a great excess of water resulting in a fatally weak structure.
Accordingly, it is a surprising concept that by adding what would ordinarily be an excessive amount of water, the resulting liquefaction enables the air promptly to percolate out, enables the binder to be distributed so as to give each region the correct amount of binder for the ultimately intended effect, for the piling to be formed without abrupt regional inconsistences, and for the resulting piling to have a known and sufficient strength from top to bottom.
The counter-intuitive concept of this invention is to accept a reduction in the unit strength of the resulting column in exchange for a resulting elimination of most of the air with its adverse consequences, thereby providing a continuous column without abrupt discontinuities between soil layers, with predictable minimum strength at every region from top to bottom, with homogeneity at more stations.
The process of this invention rotates a tool into and out of the ground, and during this action adds water and binder in a defined manner and amounts. One is entitled to ask what can be new in the combination of these simple acts. This question is answered in large part by examination of the products produced by already known processes. While all of the known processes can produce an acceptable piling for at least some applications, generally they are unattractive for many applications. The products they make in some soils and conditions are found to be deficient or unreproduceable in major properties in other soils, especially from depth to depth. It is an object of this invention to provide a process for making in-situ pilings with consistent and predictable properties from top to bottom, with known and sufficient unit strength, affordability of machinery, and reduced costs of materials, labor and capital equipment.
Some of the criteria for selecting an acceptable type of piling are how and where the column (piling) is to be made, what is its intended use, what are the properties of the soil in which it is to be formed, part of which will be used for the piling itself, what is the cost of making the piling, and what is the amount of binder needed to accomplish the desired objectives. When pilings of higher strength are desired, for example, larger quantities of binder can be used (at greater expense). The cost includes that of the very substantial machinery needed for its manufacture, the ability to place the machinery where it is needed at the site, the cost of labor to run the machinery, the speed of production, and the relative cost of the materials used to make the piling.
As to the materials, the costs of existing soil and water are negligible provided that the existing soil is in acceptable condition. The same is not true for the cost of the binder. It is the practice of most in-situ processes to use considerable excess binder to be certain that enough is present at all locations in the column. Certainty of the minimum properties of the column at all of its levels and regions could enable a designer to use only the amount of binder actually needed for suitable minimum properties throughout, plus the lowest reasonable margin of safety. If he does not have such assurance, he would normally provide an extra amount of binder, and then often still would add a wishful additional excess of it. Clearly, reduction of uncertainty can result in large savings in the cost of the binder, which is a major expense. pressure existing at different depths. This aids in providing a mixture which enables air readily to escape by percolation. It is evident that proper binder concentration at all levels is important to making an optimal column.
As to this, there is a surprising number of variables that can affect the ultimate choice of the amount of binder to be used. Perhaps the most profound variable is the consistency of the formation in which the column is to be made from top to bottom. It is not unusual for a formation within perhaps only 30 feet of depth, to include lightly consolidated material near the surface, very hard layers immediately underneath it, and perhaps a soft clay underneath them. The requirements for water and for cement can vary remarkably among these, and yet the strength of the ultimate column should be as nearly uniform as possible from top to bottom. A generally unappreciated consideration regarding the need for binder is the relative efficiency of the kinds of soil to act as aggregate that will be bound by the binder to form the ultimate solid. For example, sandy soil is very efficient in its binding with cement. Clay is very much less efficient and needs more cement. Pulverized soil is more efficient than soil fragments of hard consolidated material.
If one is to form a piling in soil in which several layers of different types of structure exist, and desires to provide enough binder to function sufficiently for all of them, he could provide binder at a constant rate suitable for the layer least efficient in its use of cement. Then the binder concentration would be the same at all depths. This can be a very significant waste, because the correct objective of the piling is not to have a constant binder concentration, but instead to have a known minimum strength along its entire length. Much of the column then could contain excess cement.
If the soil has been investigated as it should be, then the amount of binder needed at respective depths can be calculated, making significant savings. As will be shown, this is only the first of a number of criteria that can and should be considered.
The objective would seem to be to correlate the supply of binder to the need at different depths. This overlooks how the powdered (or granular) binder is supplied. It is brought to the machinery in bulk transport and held for discharge from a holding tank. Interestingly, the amount of binder being dispensed is as a practical expedient learned not from a flow-sensinq device, but from continuous weighing of the binder tank with its contents. Flow sensing devices are speedily destroyed by abrasive binder. Surprisingly accurate measurements are readily attained from the weighing operation. The diminishing weight of the tank and its contents is a sufficient measure of the dispensed binder.
The binder is conveyed from the tank through a hose extending from the tank to the top of a tower, and then down to the tool, under propulsion of a pressurized air stream. The air enters the bore along with the dry binder. There is usually at least a 40 foot flow path from the tank to the tool. The binder is fed into the air stream at a rate determined by a feed mechanism such as a star wheel located at the tank. If one were to “tailor” the flow, it means coordinating the need for a particular amount of binder at some depth remote with a feed mechanism located at least 40 feet upstream. It is not a reasonable option to place the feed mechanism any closer to the point of use. However, this can be calculated and programmed effectively to deliver a desired amount at the point of delivery, knowing the rate of flow.
An objective of this invention is to provide the resulting in-situ mix with a binder/aggregate strength that is as uniform and known as possible from depth to depth, with a fluid-like consistency, having fewest air pockets and clumps of aggregate.
This is far from a minor matter. The ability to predict the properties of the piling from top to bottom with a minimum safety factor can significantly reduce the amount of binder actually required compared to the amounts used in the wet and dry methods, and also relative to dry methods in which water is also added. It is an object of this invention to produce a column which when mixed and left to set up is in an amorphous condition resembling a very wet mud, and which cures to a piling with reasonably consistent minimum properties throughout.
In addition, by providing the binder in part in the downward tool movement and in part in the upward movement, there is an averaging out of the supply, further to assure proper amounts at the various depths.
Saving of binder has environmental consequences far beyond the expense itself. The manufacture of cement consumes a large amount of fuel, producing greenhouse gases. This invention reduces these emissions because of its use of less binder.
Such a near-homogeneous pre-set condition is not fully accomplished in the known art. Especially in clay type soils, the motive power required to accomplish mixing in the bore is sometimes so large that very powerful and excessively heavy equipment must be provided for it to work. Even these frequently stall. In addition, the weight of the equipment precludes its use on soils which will not support very heavy loads, thereby reducing the scope of useful applications of the process when equipment needed cannot be accommodated on the site. By providing a fluidized mix, the tool works against a lesser resistance.
It is an object of this invention to provide a process whose power needs can be met with the use of less powerful and lighter weight machinery, machinery so light that it can be used on soils so weak as just strong enough for a man to walk on. This is a profound advantage, for example in road building, levee repair, and railroads. The process is therefore useful in a very wide range of applications.
This invention provides yet another advantage, that of certainty of the ultimate properties of the piling. It is instructive to compare the available techniques and results of tests of various kinds of in-situ pilings.
Pilings made with the dry method rarely show consistency from top to bottom. Often regions of these pilings readily fragment. These pilings usually can not be pulled out of the ground for testing because they are so fragile that they break before they can be extracted. Also they cannot be cored, because the piling often powders, where it is cored. Pilings according to this invention can almost never be pulled out except as a body, with the use of cast-in-place rods. However they can be sampled by core drills. Thus pilings prepared by the dry method can be relied on only for properties related to their most unfavorable parts, while pilings according to this invention can be relied on for predicted, uniform properties over their entire length and importantly, can be sampled.
The properties of pilings made by the wet method frequently vary along the height of the piling, and are often wastefully produced. The soil and wastage they create is expensive.
Still another advantage of the fluidity of the mixture is that it enables the blades and also the outriggers to cut and macerate such organic material as may be present. For example, many levies have grown trees on them, and their roots may be in the situs where the piling is to be formed. The fluidity and suitable shape and size of the confronting edges of the blades and outriggers enable this material to be reduced to sizes which do not impair the properties of the cured piling.